1. Field of Invention
The present invention relates to a powering circuit of an AC-DC converter, and in particular, to a powering circuit of an AC-DC converter that turns on a switch only when an AC input voltage is lower than a preset reference voltage and sustaining a DC output voltage in a stable DC bias range.
2. Related Art
A conventional AC-DC converter adopts an isolated voltage divider design. In this design, after an AC power supply is coupled to a rectifier, an external transformer converts a high AC power supply into a low-voltage DC power supply for a low-voltage DC power supply device. However, the external transformer has a large volume, which goes against the demands for miniaturization. Then, linear regulators such as low dropout voltage regulator (LDO) are provided to solve the above problem, but the power consumption of switches of the LDO may easily cause unsatisfactory conversion efficiency.
Accordingly, the decrease of the power consumption and the improvement of the conversion efficiency have become major topics in the current miniaturized AC-DC power supply system. An AC-DC converter circuit without the use of a transformer capable of outputting a stable constant current, US Patent Publication No. 2002/0044471, has been put forward by the applicant of the present invention. This AC-DC converter circuit is applicable to a load device such as a light-emitting element (for example, an LED) in need of a stable current supply. According to the technical means, a current switching circuit is used to limit a load current within a stable range. A control circuit is used to determine the on/off state of the current switching circuit according to the potential difference between the output and input of the control circuit. When the potential difference is lower than a preset value, the control circuit switches on the load current, and when the potential difference is higher than a preset value, the control circuit switches off the load current, thereby achieving the effect of limiting the load current.
Further, U.S. Pat. No. 6,169,391 B1 has also disclosed an AC-DC converter circuit without the use of a transformer. A control circuit functioning similarly to a linear regulator is employed to limit the output voltage of a DC power supply. The control circuit has a sensing circuit for detecting an input voltage Vin of an AC power supply, and controls the on/off state of a switch according to the value of the input voltage Vin. The sensing circuit is comprised of two voltage divider resistors connected in series and a zener diode connected in series to the voltage divider resistor. However, such circuit design is obviously complex.